Commit message: ALSA: pcm: oss: Fix race at SNDCTL_DSP_SYNC

There is a small race window at snd_pcm_oss_sync() that is called from
OSS PCM SNDCTL_DSP_SYNC ioctl; namely the function calls
snd_pcm_oss_make_ready() at first, then takes the params_lock mutex
for the rest.  When the stream is set up again by another thread
between them, it leads to inconsistency, and may result in unexpected
results such as NULL dereference of OSS buffer as a fuzzer spotted
recently.

The fix is simply to cover snd_pcm_oss_make_ready() call into the same
params_lock mutex with snd_pcm_oss_make_ready_locked() variant.

Reported-and-tested-by: butt3rflyh4ck <butterflyhuangxx@gmail.com>
Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/CAFcO6XN7JDM4xSXGhtusQfS2mSBcx50VJKwQpCq=WeLt57aaZA@mail.gmail.com
Link: https://lore.kernel.org/r/20220905060714.22549-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
CVE: CVE-2022-3303
CWE: CWE-362
Vulnerability description: A race condition flaw was found in the Linux kernel sound subsystem due to improper locking. It could lead to a NULL pointer dereference while handling the SNDCTL_DSP_SYNC ioctl. A privileged local user (root or member of the audio group) could use this flaw to crash the system, resulting in a denial of service condition

Project: linux

static int snd_pcm_oss_sync(struct snd_pcm_oss_file *pcm_oss_file)
{
	int err = 0;
	unsigned int saved_f_flags;
	struct snd_pcm_substream *substream;
	struct snd_pcm_runtime *runtime;
	snd_pcm_format_t format;
	unsigned long width;
	size_t size;

	substream = pcm_oss_file->streams[SNDRV_PCM_STREAM_PLAYBACK];
	if (substream != NULL) {
		runtime = substream->runtime;
		if (atomic_read(&substream->mmap_count))
			goto __direct;
		err = snd_pcm_oss_make_ready(substream);
		if (err < 0)
			return err;
		atomic_inc(&runtime->oss.rw_ref);
		if (mutex_lock_interruptible(&runtime->oss.params_lock)) {
			atomic_dec(&runtime->oss.rw_ref);
			return -ERESTARTSYS;
		}
		format = snd_pcm_oss_format_from(runtime->oss.format);
		width = snd_pcm_format_physical_width(format);
		if (runtime->oss.buffer_used > 0) {
#ifdef OSS_DEBUG
			pcm_dbg(substream->pcm, "sync: buffer_used\n");
#endif
			size = (8 * (runtime->oss.period_bytes - runtime->oss.buffer_used) + 7) / width;
			snd_pcm_format_set_silence(format,
						   runtime->oss.buffer + runtime->oss.buffer_used,
						   size);
			err = snd_pcm_oss_sync1(substream, runtime->oss.period_bytes);
			if (err < 0)
				goto unlock;
		} else if (runtime->oss.period_ptr > 0) {
#ifdef OSS_DEBUG
			pcm_dbg(substream->pcm, "sync: period_ptr\n");
#endif
			size = runtime->oss.period_bytes - runtime->oss.period_ptr;
			snd_pcm_format_set_silence(format,
						   runtime->oss.buffer,
						   size * 8 / width);
			err = snd_pcm_oss_sync1(substream, size);
			if (err < 0)
				goto unlock;
		}
		/*
		 * The ALSA's period might be a bit large than OSS one.
		 * Fill the remain portion of ALSA period with zeros.
		 */
		size = runtime->control->appl_ptr % runtime->period_size;
		if (size > 0) {
			size = runtime->period_size - size;
			if (runtime->access == SNDRV_PCM_ACCESS_RW_INTERLEAVED)
				snd_pcm_lib_write(substream, NULL, size);
			else if (runtime->access == SNDRV_PCM_ACCESS_RW_NONINTERLEAVED)
				snd_pcm_lib_writev(substream, NULL, size);
		}
unlock:
		mutex_unlock(&runtime->oss.params_lock);
		atomic_dec(&runtime->oss.rw_ref);
		if (err < 0)
			return err;
		/*
		 * finish sync: drain the buffer
		 */
	      __direct:
		saved_f_flags = substream->f_flags;
		substream->f_flags &= ~O_NONBLOCK;
		err = snd_pcm_kernel_ioctl(substream, SNDRV_PCM_IOCTL_DRAIN, NULL);
		substream->f_flags = saved_f_flags;
		if (err < 0)
			return err;
		mutex_lock(&runtime->oss.params_lock);
		runtime->oss.prepare = 1;
		mutex_unlock(&runtime->oss.params_lock);
	}

	substream = pcm_oss_file->streams[SNDRV_PCM_STREAM_CAPTURE];
	if (substream != NULL) {
		err = snd_pcm_oss_make_ready(substream);
		if (err < 0)
			return err;
		runtime = substream->runtime;
		err = snd_pcm_kernel_ioctl(substream, SNDRV_PCM_IOCTL_DROP, NULL);
		if (err < 0)
			return err;
		mutex_lock(&runtime->oss.params_lock);
		runtime->oss.buffer_used = 0;
		runtime->oss.prepare = 1;
		mutex_unlock(&runtime->oss.params_lock);
	}
	return 0;
}